This invention relates in general to measurement of molecular weights and in particular to a method for determining the molecular weight of a substance contained in a solution.
A primary goal of the invention was to determine the average molecular weight of fulvic acids directly from aqueous solutions. Fulvic acids are found in soil and water, and they are traditionally defined according to their solubilities in water. Conventional separation and fractionation techniques can not be used to isolate individual molecular components of fulvic acids. Hence, the study of humic substances is limited to investigating bulk properties of this diverse group.
In the past, a known variety of techniques such as Gel Filtration ChromatograpHy (GFC), Vapor Pressure Osmometry (VPO), Laser-Desorption Fourier Transform ion Cyclotron Resonance Mass Spectrometry (LD FT-ICR MS), and Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (ESI FT-ICR MS) have been used to characterize fulvic acid samples. However, each technique has its own limitations. For example, in GFC method, the absolute molecular weight distributions can not be determined due to calibration difficulties. Vapor Pressure Osmometry is a colligative property; hence, the average molecular weight is affected by the extent of dissociation of fulvic acids. In LD FT-ICR MS and ESI FT-ICR MS, during the ionization and detection events, collisions and other energetic processes generally induce fragmentation and/or adduct formation.
It is also known to use pyrolysis/methylation to identify the constituents of fulvic acid samples. For example, pyrolysis reveals the presence of carboxyl groups in alipHatic and aromatic structures. Nevertheless, due to ion fragmentation and side reactions that occur during the pyrolysis process, this technique can not be used to obtain the average molecular weight of fulvic acids.
Accordingly, it would be desirable to provide a simple process for determination of the average molecular weight of fulvic acids.
This invention relates to a method for determining the molecular weight of a substance contained in a solution.
The invention involves a new approach to calculate the average molecular weight of the Suwannee River fulvic acids in solution phase. Successive disturbances of a reaction at equilibrium and unique mass balance relationships after each disturbance are used to acquire reaction parameters such as stoichiometry, molar concentrations, and molecular weights of the species present in solution/reaction.
More specifically, the invention contemplates a method for determining the molecular weight of a substance that includes providing a first solution containing the substance, the first solution being in chemical equilibrium. A first pH of the first solution is measured. Then a predetermined amount of a standard solution is added to the original solution to form a second solution. After allowing the second solution to reach chemical equilibrium, a second pH of the second solution is measured. A predetermined amount of a standard solution is added to the second solution to form a third solution and, after allowing the third solution to reach chemical equilibrium, a third pH of the third solution is measured. The first, second and third pH""s are then used to develop a set of equations that are simultaneously solved to calculate the molecular weight of the substance contained in the first solution.
It is further contemplated that the standard solution that is added can be either an acidic or an alkaline solution.
Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings.